The circumzenithal arc, CZA, is the most beautiful of all the halos.
The first sighting is always a surprise, an ethereal rainbow fled
from its watery origins and wrapped improbably about the zenith.
It is often described as an "upside down rainbow" by
first timers. Someone also charmingly likened it to "a grin
in the sky".
Look straight up near to the zenith when the sun if fairly
low and especially if sundogs
are visible. The centre of the bow always sunwards and red is on
the outside.

The CZA is never a complete circle around
the zenith, that is the exceptionally rare and only recently photographed
Kern arc.